1. Field of the Invention
The present invention relates to a semiconductor laser device formed by a gallium nitride-based semiconductor, and to an optical information reproducing apparatus using the same.
2. Description of the Background Art
In recent years, a semiconductor laser device that emits light in a range from a blue region to an ultraviolet region has been prototyped using a nitride-based semiconductor material represented by GaN, InN, AlN and a mixed crystal thereof. FIG. 19 shows a nitride semiconductor laser device oscillating at a wavelength of 405 nm, which was reported by Masaru Kuramoto et al. in Japanese Journal of Applied Physics vol. 38 (1999), pp. L184-L186.
A semiconductor laser device 500 has hexagonal nitride-based semiconductor layers (hereinafter referred to as a xe2x80x9clayered lumpxe2x80x9d) 12 formed on an n-GaN substrate 10 (film thickness of 100 xcexcm). Layered lump 12 is constituted by an n-Al0.07Ga0.93N lower clad layer (film thickness of 1 xcexcm), an n-GaN lower guide layer (film thickness of 0.1 xcexcm), an In0.2Ga0.8N (film thickness of 3 nm)/In0.05Ga0.95N (film thickness of 5 nm)-3 quantum well active layer, a p-Al0.19Ga0.81N evaporation preventing layer (film thickness of 20 nm), a p-GaN upper guide layer (film thickens of 0.1 xcexcm), a p-Al0.07Ga0.93N upper clad layer (film thickness of 0.5 xcexcm), and a p-GaN contact layer (film thickness of 0.05 xcexcm) that are layered in this order. Moreover, a positive electrode 14 and a negative electrode 15 are formed at the top and bottom sides of the layered lump, respectively. A mirror end face 400 is formed by a conventional cleaving method, which will be described later in detail. Furthermore, a stripe optical waveguide 13 is provided within layered lump 12, and serves to guide laser light. Semiconductor laser device 500 has a waveguide structure in which an active layer and a guide layer are interposed between clad layers. Thus, light emitted at the active layer is trapped into the waveguide structure, and the mirror end face functions as a laser cavity mirror, generating laser oscillating operation.
Mirror end face 400 is manufactured by the conventional cleaving method, for example, as described below. FIG. 20 illustrates the conventional cleaving method, showing an enlarged plan view of a substantial part of a wafer 121 in which hexagonal gallium nitride-based semiconductors are layered with a required layer structure on n-GaN substrate 10.
First, wafer 121 is prepared in which hexagonal gallium nitride-based semiconductors are layered with a required layer structure on an n-GaN substrate 10, and positive electrode 14 is formed. At a peripheral portion on the surface of wafer 121 (a surface opposite to n-GaN substrate 10), a groove 122 having a length of approximately 0.1 to a few millimeters is formed in a direction parallel to a cleavage plane which is unique to the material of a hexagonal nitride-based semiconductor. Here, the groove is formed by dicing or scribing, and specifically, a (1-100) plane is selected as the cleavage plane of the semiconductor layer described above.
Next, an external force is applied to n-GaN substrate 10 to divide wafer 121 into pieces in the direction parallel to the cleavage plane unique to the hexagonal nitride-based semiconductor, to obtain a plane which is to be mirror end face 400 of semiconductor laser device 500 (see FIG. 19). Here, a cleavage line 123 presents an issue. Details will be described later.
The semiconductor laser device according to the conventional technique described above has the problems as indicated below.
[Problem 1] Effects by Substrate-Side End face
Inventors of the present invention fabricated the semiconductor laser device according to the above-described conventional technique, to find one or two in every ten semiconductor laser devices that oscillate at two different wavelengths. FIG. 21 shows an oscillation spectrum of a semiconductor laser device oscillated at two different wavelengths. While vertical multimode oscillation occurred in the vicinity of threshold current, an envelope shaped by each peak is bimodal. This means that, in addition to a peak group 131 of a plurality of vertical modes around a primary oscillation wavelength, another peak group 132 of a plurality of vertical modes has occurred around a point a little toward a longer wavelength side or a shorter wavelength side. In FIG. 21, location of peak group 132, whether it is on the longer wavelength side or on the shorter wavelength side with respect to peak group 131, is not constant for each device. Peak group 132 may be located on a longer wavelength side in one semiconductor laser device, whereas it may be located on the shorter wavelength side in a different semiconductor laser device fabricated in the same lot.
The inventors of the present invention examined and found that this was due to a leaking mode to the n-GaN substrate in some ways, and proved that the leakage mode was caused by oscillation light generated within the cavity formed by a pair of n-GaN substrate portions on the mirror end face (hereinafter referred to as xe2x80x9csubstrate-side end facesxe2x80x9d).
Thus, even with semiconductor laser devices formed in the same lot, a ratio of the length of a cavity constituted by a pair of substrate-side end faces to the length of a cavity constituted by a pair of the layered lump portions on the mirror end face (hereinafter referred to as xe2x80x9clayered-lump-side end facesxe2x80x9d). Thus, relative positions of peak group 131 and peak group 132 in FIG. 21 may be different in each device. In the present specification, laser light oscillated at a wavelength different from the primary wavelength is referred to as a xe2x80x9csubstrate leaking mode.xe2x80x9d
Threshold current of the substrate leaking mode is somewhat higher than that of the primary laser oscillation mode, resulting in nonlinear I-L property of the semiconductor laser, which is undesirable in operation of the semiconductor laser device. Moreover, the substrate leaking mode emits light in the same direction as that of the primary oscillation light, which makes it impossible to separate the substrate leaking mode from the primary laser light in a spatial sense. Therefore, when such a laser device is mounted to an optical information reproducing apparatus such as an optical pickup, noise may be caused, resulting in lowering of an SIN ratio.
[Problem 2] Deterioration in Flatness of Mirror End Face
The inventors of the present invention fabricated the semiconductor laser device according to the conventional technique, and failed in some cases to obtain a good cleavage plane on the mirror end face. On mirror end face 400 of semiconductor laser device 500 shown in FIG. 19 produced according to the conventional technique, a number of vertical streaks 16 were observed within layered lump 12 of hexagonal nitride-based semiconductors, including the portion of optical waveguide 13. When observed in detail, it was found that vertical streaks 16 were concavities and convexities, i.e. surface roughness, generated across a region extending from the lower surface of n-GaN substrate 10 to the upper surface of layered lump 12. The size of each streak is evaluated along a line perpendicular to the direction of layering (the left to right direction in FIG. 19), and a RMS (Root Mean Square) value of approximately 1 to 6 nm along the length of 4 xcexcm is obtained. Three to ten such semiconductor laser devices were observed in every ten devices. Though the cause thereof is unknown, it can be interpreted as follows.
The inventors of the present invention examined and evaluated the n-GaN substrate by XRD (X-Ray Diffraction), to find that a half band width of a peak indicating the  less than 0001 greater than  direction was approximately four minutes, which bears comparison with the GaN film grown by a normal MOCVD Metal Organic Chemical Vapor Deposition) device, whereas a half band width of the peak indicating the  less than 1-100 greater than  direction was approximately 12 minutes, which was significantly large compared to the value with the GaN film (approximately 6 minutes) grown by the normal MOCVD device. Thus, it can be said that there are many n-GaN substrates that are insufficient in their a-axis orientation, and therefore cleaving of an n-GaN substrate would not result in a flat cleaved mirror end face, generating a number of vertical streaks. Furthermore, the layered lump of hexagonal nitride semiconductors is directly formed on the surface of an n-GaN substrate in the conventional semiconductor laser device, so that stress from the n-GaN substrate is propagated to the layered lump when the mirror end face of the semiconductor laser is formed by the conventional cleaving method, preventing flat cleaving. This is considered to be a cause of deterioration in flatness on the layered lump side of the mirror end face.
Moreover, when the surface of the n-GaN substrate was observed by a cathode luminescence measurement, a number of dark points or dark lines were found scattered on the surface. These indicate crystalline defects distributed on the substrate surface, which occurred at alleviation of a residual stress applied when the substrate surface was finished by mechanical polishing. In substrates of other materials, for example, in a GaAs substrate, isotropic chemical etching is performed using an etchant on the substrate surface after mechanically polished, so as to remove the residual stress applied from the substrate surface to the depth of several tens of microns. Whereas, an appropriate etchant is absent in the n-GaN substrate, resulting in incomplete removal of the residual stress applied from the substrate surface to the depth of several tens of microns. Such a layer in which the residual stress is applied through the depth of several tens of microns from the substrate surface will be referred to as a xe2x80x9cresidual stress layer.xe2x80x9d In the residual stress layer, a cleavage plane that is unique to the hexagonal nitride-based semiconductor is present, which however forms a curved surface with a warp due to the residual stress, rather than a flat surface. When a wafer is fabricated by directly forming a layered lump of hexagonal nitride-based semiconductors on the surface of a substrate including such a residual stress layer near the substrate surface, the layers in the wafer are arranged in the order of, from the substrate surface, the layered lump, the residual stress layer and the n-GaN substrate. When such a wafer is cleaved, a cleavage plane unique to the hexagonal nitride-based semiconductor is warped between the layered lump and the n-GaN substrate. This is also considered as a cause of the failure in obtaining a flat cleaved end face on the layered-lump side.
As such, according to the conventional technique, only a surface with roughness could be obtained despite of the mirror end face formed in parallel with the cleavage plane unique to the hexagonal nitride-based semiconductor. This causes not only variation in the device property such as a threshold or differential efficiency due to variation of a mirror reflectance, but also deterioration in the optical property due to irregularity on a light emitting plane such that FFP (Far Field Pattern) does not form a smooth unimodal shape, resulting in split peaks or a ripple. Such an abnormality in FFP is undesirable since it would cause, particularly in an application to an optical pickup and the like, insufficient convergence or, in an extreme case, stray light.
Furthermore, cracks or roughness on the mirror end face tends to occur on an upper surface side (the layered lump side, not on the n-GaN substrate side). Because the resistance of a p-side clad layer is high in a nitride-based semiconductor laser device, an active layer is located very close to the surface in order to reduce the device voltage, and the depth of the active layer is generally equal to or lower than 1 xcexcm. The fact that the cracks or roughness likely occur on the upper surface side means that the waveguide portion may be affected with high chances, lowering the yield in manufacturing of devices.
[Problem 3] Linearity of Cleavage Line
The inventors of the present invention fabricated the semiconductor laser device according to the above-described conventional technique, to find a number of polygonal cleavage lines 123, as shown in FIG. 20, extending in directions different from a desired direction. Though the cause thereof is unknown, it can be interpreted as described below.
When layered lump 12 of hexagonal gallium nitride-based semiconductors having a required layer structure is grown on n-GaN substrate 10, roughness of concavities and convexities each having a shape of a six-sided pyramid are formed on the surface of layered lump 12. Specific measurements showed that the average surface roughness (i.e. concavities and convexities) across a region extending for the length of 1 mm in the direction of a surface perpendicular to the layering direction is Ra approximately 400 to 500 xc3x85. Such roughness is formed when a residual stress layer is present near the surface of the n-GaN substrate. When a wafer on which such roughness occurred is cleaved by the conventional cleaving method, cleavage line 123, which is supposed to be a straight line parallel to the left-to-right direction in FIG. 20, is cleaved bending at an angle of approximately 60 degrees with respect to the straight line, affected by the roughness on the surface of the layered lump. As a result, the cleavage line extends in a direction different from a desired direction.
Such a condition in which a polygonal cleavage line is easily formed causes a problem in that the mirror end face of the semiconductor laser device is inclined rather than being perpendicular to the substrate surface. This also means that the mirror end face is not perpendicular to the stripe of the optical waveguide, causing raise in the threshold current and variation of laser light emitting direction due to lowering of light reflectance on the mirror end face, resulting in a lowered yield in manufacturing of devices. In addition, even if the mirror end face happens to be perpendicular to the stripe of the optical waveguide, variation still occurs in the length of the optical cavity, causing variation of threshold current or operation voltage, also resulting in a lowered yield in manufacturing of devices.
The present invention is directed to solve the problems 1 to 3 described above, to provide a nitride semiconductor laser device optimal for an application to a light pick-up and the like with a good yield, and to realize an optical information reproducing apparatus having a superior light condensing property.
In order to achieve the objects described above, according to one aspect of the present invention, a semiconductor laser device includes a first semiconductor layer including a gallium nitride substrate; a second semiconductor layer including a hexagonal nitride-based semiconductor, having an active layer and provided on an upper side of the first semiconductor layer; and mirror end face formed by cleavage such that both of the first semiconductor layer and the second semiconductor layer have side surfaces exposed onto an approximately same plane. An average roughness of an exposed portion of the second semiconductor layer is at most a half of an average roughness of an exposed portion of the first semiconductor layer, on the mirror end face.
The structure above allows a reflectance to be lower in the region on the first semiconductor layer side compared to that in the region on the second semiconductor layer side, on the mirror end face formed by cleaving, so that a substrate leaking mode can be suppressed. Particularly, in order to suppress the substrate leaking mode and to form a unimodal envelope shaped by a vertical mode peak of an oscillation spectrum, the measurement value on the second semiconductor layer side end face must be at most a half of the measurement value on the first semiconductor layer side end face when compared for RMS of roughness across a region extending for 4 xcexcm in the direction perpendicular to the layering direction. The above structure satisfies the condition, so that the envelope can have the unimodal shape.
Preferably, the semiconductor laser device further includes a buffer layer provided between the first semiconductor layer and the second semiconductor layer. By such a configuration that the buffer layer is interposed, cleaving of the second semiconductor layer side is independently performed, unaffected by the stress at cleaving of the first semiconductor layer side, so that a highly flat plane with few streaks can be obtained, and thus a precise semiconductor device with reduced variation in reflectance and few cracks can easily be manufactured. Furthermore, with the buffer layer interposed between the first semiconductor layer, the surface of the second semiconductor layer, when layered, is unaffected by the residual stress layer on the surface of the semiconductor layer, so that occurrence of the surface roughness on the second semiconductor layer can be suppressed. As a result, the second semiconductor layer can be prevented from being cleaved in an undesired direction due to effects of surface roughness, facilitating manufacturing of a semiconductor device that is accurately cleaved in the right direction.
Preferably, the buffer layer includes InxAlyGa1-x-yN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61). Such an structure allows the use of the same nitride semiconductor as that of an upper and lower semiconductor layers, so that the resistance of the buffer layer can be reduced. Moreover, the difference in thermal expansion coefficients between the buffer layer and the upper and lower adjacent semiconductor layers can also be reduced, so that the reliability of the semiconductor layer device can be improved.
Preferably, the thickness of the buffer layer is at least 10 nm and at most 10 xcexcm. The structure allows an average surface roughness Ra to be 100 xc3x85 or lower when a layered lump of hexagonal nitride-based semiconductors is formed on the buffer layer as the second semiconductor layer.
Preferably, the second semiconductor layer includes a linearly formed optical waveguide, the gallium nitride substrate has a linear recess on an upper surface of the gallium nitride substrate, and the optical waveguide is arranged so as to be located above the recess. By such a structure, the second semiconductor layer grows in sequence, affected by the stepped shape of the recess, so that effects of crystalline defects of the first semiconductor layer cannot be exerted on the optical path located above the recess, reducing crystalline defects in the optical path.
According to another aspect of the present invention, a method of manufacturing a semiconductor laser device of the present invention includes a buffer growing step growing a buffer layer on an upper side of a gallium nitride substrate at a first temperature; an upper side semiconductor growing step growing a hexagonal nitride-based semiconductor layer on an upper side of the buffer layer at a second temperature higher than the first temperature, to obtain a wafer; and a wafer dividing first step dividing the wafer along a plane parallel to a cleavage plane of the hexagonal nitride-based semiconductor layer, to obtain a wafer piece.
By employing the method, even with insufficient a-axis orientation of the gallium nitride substrate, the buffer layer can suppress propagation of effects of the stress from the gallium nitride substrate to the hexagonal nitride-based semiconductor layer at dividing of a wafer into wafer pieces by cleaving, so that a divided wafer piece with a flat cleavage plane can be obtained at least in a region on the side of the hexagonal nitride-based semiconductor layer. Therefore, when the cleavage plane is to be a mirror plane, the portion of the hexagonal nitride-based semiconductor layer can be a flat mirror end face, and thus a cavity structure having a high reflectance can be obtained.
Preferably, the wafer dividing first step includes a first groove forming step forming a groove extending in a dividing direction at an end of a surface on the hexagonal nitride-based semiconductor layer side of the wafer, before dividing the wafer. Such a method allows cleaving to be generated in a desired direction on the second semiconductor layer by forming a groove of a limited length, so that the resulting mirror plane can have a required roughness.
Preferably, the method of manufacturing a semiconductor laser device further includes a wafer dividing second step dividing the wafer piece along a plane non-parallel to the cleavage plane of the hexagonal nitride-based semiconductor layer. By the method, the semiconductor laser device can be obtained by dividing the wafer piece, and the obtained semiconductor laser device can set its cleavage plane as a mirror end face, facilitating attainment of a cavity structure having a high reflectance.
Preferably, the wafer dividing second step includes a second groove forming step forming a groove extending in a dividing direction so as to traverse a surface on a gallium nitride substrate side of the wafer piece from one end to the other end, before dividing the wafer piece. The method allows linear dividing at a desired position.
According to a further aspect of the present invention, an optical information reproducing apparatus includes any one of the semiconductor laser devices described above as a light source, and converts reflection light of laser light emitted from the light source onto an optical disk having an information recording plane, to reproduce information recorded on the optical disk. By such a structure, a semiconductor laser device with a flat mirror end face on the second semiconductor layer side, i.e. on the layered lump side, and with a good FFP is used, so that light can be condensed onto the information recording plane of the optical disk with high resolution. Moreover, as a semiconductor laser device is used in which oscillation of the substrate leaking mode is suppressed so that the envelope shaped by the vertical mode peak of an oscillation spectrum is unimodal, information can be read at a low bit error rate from the optical disk on which information is recorded with high density.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.